The use of structural pillars made from a concrete material is well known and widely practiced in the construction industry. Such pillars are typically poured into a tubular pillar form made of spirally wrapped paper, although other prefabricated pillar forms are well known and commonly used for this purpose. According to most building codes, structural pillars must be supported by a footing located below the level of maximum frost penetration and usually set on a coarse aggregate bed to ensure adequate drainage. The footing which is normally also made of concrete material provides support for the pillar and its load. Traditionally, wooden footing forms built on site were used. More recently, prefabricated forms have been introduced, which overcome the problems encountered with wooden forms, such as the need for at least one cross-piece for supporting the tubular pillar form, the labour intensive and time consuming assembly and disassembly of the wooden forms, improper filling when concrete is fed through the top of the tubular form, and the need to wait until the footing is set before backfilling.
Various types of prefabricated footing forms exist, most of which are somewhat tapered towards the top where the pillar form is adjoined. Bell-shaped (Joubert, U.S. Pat. No. 4,830,543), and conical (Jackson, U.S. Pat. No. 3,108,403; Miller U.S. Pat. No. 1,296,995; Gebelius, U.S. Pat. No. 4,648,220) or frusto-conical (wells, U.S. Pat. No. 4,673,157; Nagle, U.S. Pat. No. 5,271,203) forms are known, with the latter being most common. A conical shape facilitates proper filling of the form with concrete material, makes the form stable and able to support the pillar form, and sometimes even allows for backfilling prior to pouring of the concrete material. However, tapered prefabricated forms have certain structural limits. Swinimer (U.S. Pat. No. 5,785,459) discloses that in order to achieve complete filling of a conical form without vibrating the concrete material, the pitch of the sidewall must be between about 45° and about 65°. Such a sidewall angle is impractical for industrial size applications with large footprint (bottom diameter), for example above 30 inch diameter, since it will lead to an impractically high form and high material cost. The higher the footing, the deeper it must be buried to remain below frost level. Moreover, the transition region between the footing and the pillar, which is a peak stress point of the pillar/footing structure should be located as far below grade as possible to reduce the lateral load at this transition region. Thus, since the vertical location of this transition region is governed by the height of the footing form, forms of large footprint and a sidewall angle of 45° or above are impractical and uneconomical due to high installation and/or excavation cost. Consequently, a more economical and practical prefabricated form is desired.